Generally, a thermal recording material comprises, on a support, a heat-sensitive recording layer containing, as main components, an electron-donating dye precursor, which is usually colorless or light-colored, and an electron-accepting compound. By application of heat to such a thermal recording material with a thermal head, a thermal stylus, laser beam or the like, an instant reaction between the electron-donating dye precursor and the electron-accepting compound occurs and thereby a recorded image is produced. Such a thermal recording material is advantageous, for example, in that records can be made thereon with a relatively simple device ensuring easy maintenance and no noise generation. Therefore, thermal recording materials are widely used for a measuring recorder, a facsimile, a printer, a computer terminal, a label printer, a ticket machine for passenger tickets or other tickets, and the like. Particularly in recent years, thermal recording materials are used as financial records such as receipts of gas, water, electricity and other bill payments, billing statements issued from ATMs at financial institutions and various receipts, thermal recording labels or tags for point of sales (POS) system, etc.
As the application of thermal recording materials becomes more diverse, not only texts but also gray scale images are printed more frequently, and thus thermal recording materials that are excellent in dot reproducibility in any print density, whether low or high, free from occurrence of partially missing print and excellent in recorded image quality are desired. With the increase in the speed of recording devices, thermal recording materials that excel in thermal responsiveness are also desired.
For improvement in recorded image quality of thermal recording materials, methods for enhancing smoothness of the heat-sensitive recording layer surface to strengthen adhesion to a thermal head have been developed. For example, Patent Literature 1 describes surface processing of a heat-sensitive recording layer to give a Bekk smoothness of 200 to 1000 seconds. Another proposed solution for improvement in recorded image quality is to produce a heat-insulating intermediate layer between a support and a heat-sensitive recording layer to enhance thermal responsiveness of thermal recording materials. For example, Patent Literature 2 describes an intermediate layer containing an oil-absorbing pigment, Patent Literature 3 describes an intermediate layer containing minute hollow spherical particles, and Patent Literature 4 describes a 3- to 200-μm-thick intermediate layer having minute hollow spaces and a void ratio of 50 to 95%. Further, methods for enhancing smoothness of the intermediate layer surface to attain uniform coating thickness of a heat-sensitive recording layer, which leads to reduction in print density nonuniformity resulting from nonuniform coating thickness of a heat-sensitive recording layer have also been developed. For example, Patent Literature describes blade coating using a coating liquid having a specific viscosity for formation of an intermediate layer, Patent Literature 6 describes producing an intermediate layer having minute hollow spaces and a Bekk smoothness of 2000 seconds or more, Patent Literature 7 describes thermal calendering of an intermediate layer having hollow resin particles, and Patent Literature 8 describes producing two or more intermediate layers and a heat-sensitive recording layer having a thickness standard deviation of a certain level or lower.
The methods as described above, for enhancing smoothness of a heat-sensitive recording layer, or for producing a heat-insulating intermediate layer and enhancing its smoothness, improve the recorded image quality, but their improving effects are still unsatisfactory. Therefore, desired are thermal recording materials that have an excellent recorded image quality with a uniform color density even in a halftone range, and a method for producing the same.